An EMI gasket is a conductive interface material, which is used to connect an electrically conductive shield with a corresponding section of an electrical ground, such as a ground trace of a printed circuit board (PCB). Preferably, an EMI gasket should be highly electrically conductive and conformal. The gasket material fills the gaps between the shield and ground trace, thereby shielding the enclosed electronic equipment from electromagnetic interference.
In the area of EMI gaskets, it is desirable for designers to have an EMI gasket that is narrow. This allows the designer to make the corresponding PCB ("Printed Circuit Board") ground trace narrower, and thereby pack more SMT electrical components into a given space. A typical desired width for such a narrow EMI gasket, would be approximately 0.75 mm.
Further, an EMI gasket must also have conformability to be fully functional. In another words, the EMI gasket must be compressible so that it may absorb the loose flatness tolerances that are present between a PCB and shielding cover, and shield effectively. Typically, a PCB and shielding cover will require approximately 0.2-0.3 mm of conformability out of the gasket material, to accomplish this. Since most EMI gaskets compress 25-50%, a typical desired gasket thickness (prior to compression) is roughly 0.75 mm.
Therefore, since it is desirable to have an EMI gasket's width as 0.75 mm and its thickness of 0.75, the EMI gasket would have a so-called 1:1 (one-to-one) aspect ratio.
However, when EMI gaskets are made with a 1:1 aspect ratio such as this, several problems may occur, especially in the installation process.
Presently, EMI gaskets are almost exclusively installed directly onto a conductive shield, as opposed to the PCB. The present manufacturing techniques for installing EMI gasket parts include the following: dispensing a conductive paste or a conductive liquid material directly onto a conductive shield's surface and curing the dispensed material in-situ; die-cutting a conductive sheet material having an adhesive backer and then transferring, positioning and adhering the dimensioned material directly to a conductive shield's surface; or mechanically fastening a conductive spring-like metal material to a conductive shield's surface. Typically, when these EMI gasket materials are close to 1:1 aspect ratio, they are difficult to manufacture and install.
As is well understood by those skilled in the art, SMT (Surface Mount Technology) machines typically utilize a vacuum head on the end of a high-speed pick-and-place system to install tape-and-reel-fed PCB components onto surface-mount pads on a PCB. These pads are usually pre-screened with solder-paste (or conductive adhesive) and then sent through a reflow process (such as infrared--IR, vapor-phase, or convection) to melt the solder joints (or cure the adhesive), thereby forming an electrical and mechanical connection. As stated before, EMI gaskets are generally installed directly onto the shielding cover, which mates to a matching ground trace on the PCB. However, there is an SMT-compatible EMI gasket that can be placed directly onto the PCB, and soldered (or bonded) to the ground trace. This gasket can also be soldered or bonded to the shielding cover, as well, for additional flexibility. This type of gasket goes under the trade name of GORE-SHIELD.RTM. SMT EMI gaskets, as described in co-pending U.S. application Ser. No. 09/052,080, incorporated herein by reference. A typical SMT-compatible gasket solution would be made up of many individual, discrete parts, depending upon the size of the shield required.
In developing an EMI gasket part that is compatible with the SMT process, two problems have been encountered; these problems are caused by the instability of the EMI gasket part during pick-and-place installation processes.
The first major problem in the installation process is created by the 1:1 aspect ratio of the part. The problem with the EMI gasket part occurs during the "pick-and-place" operation, which is related to the packaging method of the SMT parts. SMT parts are typically provided in either tape-and-reel format, or as loose pieces in bulk to be vibratory bowl-fed, for example. The tape-and-reel packaging contains formed pockets that are custom made to fit the dimensions of the SMT parts. But because of tolerances, the pockets must be made slightly larger than the SMT parts to ensure that parts do not stick in the pockets during subsequent removal with low suction-force vacuum nozzles on the SMT equipment. Since the EMI gasket parts mentioned above are more or less square in cross-section (due to this 1:1 aspect ratio), they therefore have a tendency to roll over if the pocket is too loose. This rollover can occur during original packaging of the parts into the pocket, shipping due to vibration or rough handling, cover tape removal, or during the subsequent vacuum nozzle pickup process. Since a single part placed in a rotated configuration can render a PCB non-functional, rollover is a serious problem.
Also just as serious, some SMT automation vision systems have had difficulty distinguishing a part that is correctly oriented once it has been picked from the carrier tape pocket. This second problem means that the vision safety-check, which discards misoriented parts before they are placed incorrectly, may fail. This reduces the overall reliability of the SMT solution dramatically.
It would be advantageous to provide an improved EMI gasket part directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided, including features more fully disclosed hereinafter.